perf hists browser: Add NULL pointer check to prevent crash

Before this patch we can trigger a segfault by following steps:

 Step 0: Use 'perf record' to generate a perf.data without callchain

 Step 1: perf report

 Step 2: Use UP/DOWN to select an entry, don't press 'ENTER'

 Step 3: Use '/' to filter symbols, use a filter which returns
         empty result

 Step 4: Press 'ENTER' (notice here that the old selection is still
		        there. This is another problem)

 Step 5: Press 'ENTER' to annotate that symbol

 Step 6: Press 'LEFT' to go out.

 Result: segfault:

 perf: Segmentation fault
 -------- backtrace --------
 /home/wangnan/perf[0x53e568]
 /lib64/libc.so.6(+0x3545f)[0x7fba75d3245f]
 /home/wangnan/perf[0x537516]
 /home/wangnan/perf[0x533fef]
 /home/wangnan/perf[0x53b347]
 /home/wangnan/perf(perf_evlist__tui_browse_hists+0x96)[0x53d206]
 /home/wangnan/perf(cmd_report+0x1b9f)[0x442c7f]
 /home/wangnan/perf[0x47efa2]
 /home/wangnan/perf(main+0x5f5)[0x432fa5]
 /lib64/libc.so.6(__libc_start_main+0xf4)[0x7fba75d1ebd4]
 /home/wangnan/perf[0x4330d4]

This is because in this case 'nd' could be NULL in
ui_browser__hists_seek(), but that function never checks it.

This patch adds checker for potential NULL pointer in that function.
After this patch the above steps won't segfault.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1449455746-41952-3-git-send-email-wangnan0@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>